(a) Field of the Invention
The present invention relates in general to a scanning electron microscope for obtaining a two-dimensional scanning image showing a shape, a composition or the like of a surface of a sample by scanning the surface of the sample to be inspected with an electron beam, and more particularly to a scanning electron microscope which is suitable for obtaining a scanning image having high resolution in a low acceleration voltage region.
(b) Description of the Prior Art
In a scanning electron microscope, a two-dimensional scanning image is obtained in such a way that electrons which have been emitted from an electron source of a heating type or a field emission type are accelerated so as to be formed into a slender electron beam (a primary electron beam) through an electrostatic or electromagnetic lens, a surface of a sample to be observed is scanned with the resultant primary electron beam using a scanning deflector, a secondary signal is detected which is formed by secondary electrons or reflected electrons which are secondarily generated from the sample by irradiation with the primary electron beam, and the intensity of the detected signal is made an input for the intensity modulation in a cathode ray tube (CRT) which is scanned synchronously with the scanning of the primary electron beam. In the general scanning electron microscope, the electrons which have been emitted from the electron source are accelerated in a space between the electron source, to which a negative electric potential is applied, and an anode electrode at ground electric potential, and the surface of the sample to be inspected at ground electric potential is scanned with the electron beam.
As the result of the fact that the scanning electron microscope has become used in the process of the manufacture of semiconductor devices or in the inspection after completion thereof (e.g., the size measurement and the inspection of the electrical operation thereof using the electron beam), there has been required a high resolution of 10 nm or less at a low acceleration voltage of 1,000V or lower with which an insulating material can be observed without being charged with electricity.
The primary factor which impedes attaining of high resolution in the low acceleration voltage region is the blur of the electron beam due to the chromatic aberration which results from the dispersion in the energy of the electron beam emitted from the electron source. In a scanning electron microscope of the low acceleration voltage type, in order to reduce the blur due to the chromatic aberration, there is mainly employed an electron source of a field emission type in which the dispersion in the energy of the emitted electron beam is small. However, even in the case of an electron source of the field emission type, the space resolution at a voltage of 500V is limited to the range of 10 to 15 nm. This does not fulfill the requirement by users.
As for the measure of solving the above-mentioned problem, there is known a method wherein an acceleration voltage of primary electron beam between an electron source and an anode electrode at the grounding electric potential is set to a value higher than the final acceleration voltage, and the primary electron beam is decelerated in a space between an objective lens at ground electric potential and a sample to be inspected to which a negative electric potential is applied, whereby the acceleration voltage of interest is set to the final low acceleration voltage (refer to Proceedings of IEEE 9th Annual Symposium on Electron, Ion and Laser Technology, pp. 176 to 186).
The effects which are provided by this method were previously confirmed by the experiments relating thereto. However, that technology has hardly been adopted commercially since the secondary electrons are drawn into an evacuable enclosure through the retarding electric field so that they are difficult to detect; and a specimen stage having high electrical insulating characteristics is required because a high voltage is applied to the sample.